1. Field of the Invention
The present invention relates to a catalyzed hydrocarbon trap material and a method of making the same. More particularly, the invention relates to such material comprising a palladium component dispersed on a refractory inorganic oxide support such as alumina, and a zeolite having temperature-responsive hydrocarbon-adsorbing and desorbing properties, and having a silver component dispersed thereon to enhance the hydrocarbon-adsorbing properties.
2. Related Art
The treatment of internal combustion engine exhaust to convert noxious components such as hydrocarbons ("HC"), carbon monoxide ("CO") and nitrogen oxide ("NO.sub.x ") to innocuous components (water, carbon dioxide and nitrogen) is, of course, well known in the art. Such conversion is attained by contacting the engine exhaust with one or more catalysts, usually comprising an oxidation catalyst and a reduction catalyst, or a so-called three-way conversion catalyst, which has the capability of substantially simultaneously oxidizing HC and CO to water and carbon dioxide, and reducing NO.sub.x to nitrogen. Such catalysts often comprise a platinum group metal such as platinum, platinum plus rhodium or palladium dispersed on a refractory inorganic oxide support such as gamma alumina. It is also known to utilize zeolite particles, including zeolite particles which have been ion-exchanged with metal cations, as a hydrocarbon trap material.
A persistent problem in meeting ever more stringent government regulations concerning the discharge of pollutants from engine exhaust is the fact that catalysts, especially oxidation catalysts, require an elevated temperature, usually above 200 or 250.degree. C., in order to attain reasonably high conversion efficiencies. Therefore, during an initial start or other period of engine operation while the engine is cold, referred to as a "cold operation period", conversion of pollutants, especially hydrocarbons, is carried out with a low efficiency, if at all. Thus, a very substantial proportion of the total oxidizable pollutants, largely comprising hydrocarbons, discharged to the atmosphere during a given period of operation, is discharged during the cold operation period. In order to ameliorate this problem, the art is aware of the expedient of using, in conjunction with the catalyst, a hydrocarbon trap material, such as certain zeolites, which will adsorb hydrocarbons at a low temperature at which the oxidation catalyst is relatively ineffective, and desorb the hydrocarbons only at a more elevated temperature, at which conversion efficiency of the oxidation catalyst is higher than during the cold operation period.
One difficulty with such prior art expedients is that such zeolite materials tend to begin desorbing hydrocarbon, and thus releasing it to the catalyst, before the catalyst is hot enough to attain acceptably high conversion efficiencies. That is, the prior art inclusion of zeolites, while improving the situation by adsorbing hydrocarbons for a period, commence the desorption too soon after the cold operation period, thereby releasing the hydrocarbons before the catalyst is sufficiently heated, so that only a limited benefit is attained. It would therefore be desirable to have a composition which adsorbs or otherwise traps and retains the hydrocarbons and does not release them until release temperatures higher than those heretofore attainable are reached by the catalyst used to oxidize the hydrocarbons.
U.S. Pat. No. 4,683,214, issued on Jul. 28, 1987 to P. J. Angevine et al ("the '214 Patent"), discloses crystallizing a zeolite from a forming mixture containing an organic template, treating the zeolite to remove the template, and blending the zeolite with a binder, for example, an inorganic oxide such as alumina, and a solution containing noble metal. The blended mixture can then be extruded to form the desired type of catalyst particle (column 2, lines 28-39). At column 2, line 48 et seq, the catalysts of the '214 Patent are said to be useful in any process or combination of processes which employ metal catalyst components such as platinum or palladium, as well as other noble metals. Processes such as hydrogenation, isomerization, reforming and others, are mentioned; at column 10, lines 63-66, the catalysts are also stated to be advantageously employed under oxidation reaction conditions as oxidation or combustion catalysts, for example, in processes such as methanol synthesis, Fischer-Tropsch process, etc.
At column 3, lines 28-33, the '214 Patent states that the noble metals include platinum, palladium and silver, among numerous others, and that, in addition, any suitable hydrogenation component, e.g., a group VIII metal, may be incorporated within a zeolite. There is no specific disclosure of combining two or more of the metals. At column 8, line 53, to column 9, line 9, the '214 Patent states that contacting a mixture of zeolite and inorganic oxide binder such as alumina with an anionic noble metal-containing species would be expected to result in noble metal loading of the binder rather than the zeolite, owing to exchange of the platinum anion with residual hydroxyl anions of the binder. The use of noble metals in both cationic and anionic form is disclosed, for example, in, respectively, claims 24 and 26 of the '214 Patent.
Another difficulty with prior art expedients in which the zeolites are ion-exchanged with cations such as silver, copper or other metals, is the perceived need to segregate catalytic components, such as platinum group metals dispersed on an inorganic oxide support material, from metal ions such as silver, which are ion-exchanged or otherwise dispersed on the zeolite particles. This requires additional manufacturing steps and segregated operations in which impregnation of the oxide supports with catalytic metals such as platinum, or platinum and rhodium, is segregated from operations in which metal cations are ion-exchanged into the zeolites. The ion-exchanged zeolite particles and the separately prepared platinum group metal-impregnated support material particles are then admixed with each other, or disposed on separate portions of a substrate, in order to provide the finished hydrocarbon trap/catalyst material.
U.S. Pat. No. 5,244,852, issued on Sep. 14, 1993 to I. M. Lachman et al ("the '852 Patent") discloses in Example I thereof combining a molecular sieve such as silicate zeolite with pseudoboehmite, a form of alumina (column 6, lines 20-48), making a slurry of the mixture, coating the slurry onto a honeycomb substrate and firing the coated substrate at a temperature of 500 to 600.degree. C. The fired substrate is then loaded with noble metal, e.g., platinum, by impregnation with a solution of noble metal to provide a three-way conversion catalyst for oxidizing HC and CO and reducing NO.sub.x in automotive engine emissions control. Subsequent examples show the use of silicalite, zeolite and ultra-stable Y-zeolite in combination with the pseudoboehmite, and silicalite zeolite in combination with alumina. In Example VI at column 8 of the '852 Patent, palladium is used as the noble metal impregnated into the silicalite zeolite-alumina washcoat. Other examples show the use of platinum or a combination of platinum and palladium impregnated into the washcoat. Still other examples show the use of mordenite zeolite with dispersible pseudoboehmite. The '852 Patent teaches that by utilizing zeolites plus a binder, such as an alumina binder, rhodium may be replaced by palladium and/or platinum in the catalyst system. Any one of the many zeolites disclosed at column 2, line 13 et seq of the '852 Patent, including ZSM-5 and Beta zeolite, is stated to be usable. Preferably, the zeolites and an alumina binder, such as gamma alumina or a precursor thereof, such as pseudoboehmite alumina, is utilized. At column 2, lines 51-59, the '852 Patent states that, "Interestingly, and fortuitously, the noble metal precursors do not substantially site on the already present zeolite. This advantageous result provides a segregated system, . . . " That is, a system wherein (column 2, line 58 et seq) the noble metals are dispersed only on metal oxide particles and the oxide and molecular sieve particles are juxtaposed to each other, and therefore each is able to catalyze the appropriate reactions. This segregated aspect, in which the noble metal (e.g., platinum or palladium) is sited only on the high surface area oxide (e.g., alumina) is reflected in the claims. For example, claim 1 of the composition claims of the '852 Patent requires that "substantially all of the noble metal is sited on the high surface area oxide . . . " The method claims of related U.S. Pat. No. 5,292,991, like the composition claims of the '852 Patent, call for the zeolite to be selected from the group consisting of ZSM zeolite and silicalite, and to be impregnated with a noble metal selected from the group consisting of platinum and palladium, and that the washcoat is characterized in that "substantially all of the noble metal is sited on the high surface area oxide . . . "U.S. Pat. No. 5,292,991, issued Mar. 8, 1994 to I. M. Lachman et al, is related to the '852 Patent as having issued on a divisional of the application which resulted in the '852 Patent.
NGK Insulators' Japanese patent document (Kokai) 8-10566 (1996) was published on Jan. 16, 1996 based on Japanese Patent Application 6-153650, filed on Jul. 5, 1994 and entitled "A Catalyst-Adsorbent For Purification Of Exhaust Gases And An Exhaust Gas Purification Method." The Abstract of this document, below referred to as "the '650 Application", discloses a catalyst-adsorbent in which a catalyst material effective for decreasing CO, HC and NO.sub.x in internal combustion engine exhaust is combined with an adsorbent material that traps hydrocarbon during cold discharge start-ups. The catalyst material may be one or more of platinum, palladium and rhodium, preferably palladium, carried on a heat-resistant inorganic oxide, e.g., activated alumina, and containing at least 2 to 30 weight percent palladium. The catalyst-adsorbent is carried onto a monolithic honeycomb and the proportion of catalyst material to adsorbent material is given as 15 to 90% catalyst and 10 to 85% adsorbent material, by weight.
In a preferred embodiment of the '650 Application, palladium in a concentration of 2 to 30 weight percent is carried on a refractory inorganic oxide which may comprise activated alumina, zirconia, silica or titania. The composition may also include rare earth oxides, except that when rhodium is employed, the addition of ceria is to be avoided.
The adsorbent material of the '650 Application is comprised of particles of primarily zeolite, preferably zeolite having a silica-to-alumina ratio of 40 or more; these include ZSM-5 and Beta zeolites. The zeolite adsorbent may optionally have metal ions dispersed therein, the presence of ions of high electro-negativity being said to increase the HC adsorptive capacity. Such ions include silver, palladium, platinum, gold, nickel, copper, zinc, cobalt, iron, manganese, vanadium, titanium, and aluminum. The '650 Application discloses that metal cations may be applied to the zeolite by either ion exchange or immersion methods. The presence in the zeolite of at least one ion of elements of Group IB of the Periodic Table (copper, silver, gold) is said in the '650 Application to manifest a high adsorptive capacity for hydrocarbons even in the presence of water. Copper and silver are stated to be preferable and silver ions, which are specified in some of the examples of the '650 Application, exchanged into the zeolite are said to be particularly desirable for adsorbing HC at higher temperatures. It is stated that the ion content of the zeolite should be greater than 20%, and preferably greater than 40%, relative to the aluminum atoms in the zeolite. Example 95 of the '650 Application shows palladium on ceria-stabilized alumina in a first coating layer and silver- and copper-exchanged ZSM-5 in a second coating layer. Table 6 shows ZSM-5 zeolites exchanged only with silver. The '650 Application discloses that in order to improve low-temperature ignition characteristics to the maximum, it is desirable to form a palladium surface coat layer in which palladium is the only noble metal carried on the particles. The '650 Application further discloses the desirability of forming a first coating on a substrate comprised of the adsorbent zeolite material, over which a second coating comprised of a catalyst material containing only palladium catalyst particles is placed. This is stated to provide excellent durability and low-temperature ignition characteristics of the catalyzed trap material.
In preparing the materials of the '650 Application, alumina powder, sometimes stabilized against thermal degradation in the known manner by impregnation with ceria, is coated with either palladium, platinum or rhodium. The zeolite materials of the '650 Application, optionally having metal ions dispersed therein, are prepared separately from the platinum group metal catalytic materials and the separately prepared powders are then applied to substrates to form the desired compositions.